Fix Field arguments variance (#573)

* Fix Field arguments variance

* fixup! Fix Field arguments variance

Test field can be used as bsse type

django-stubs/db/models/fields/__init__.pyi

@@ -38,9 +38,9 @@ _ErrorMessagesToOverride = Dict[str, Any]
 
 _T = TypeVar("_T", bound="Field")
 # __set__ value type
-_ST = TypeVar("_ST")
+_ST = TypeVar("_ST", contravariant=True)
 # __get__ return type
-_GT = TypeVar("_GT")
+_GT = TypeVar("_GT", covariant=True)
 
 class Field(RegisterLookupMixin, Generic[_ST, _GT]):
     _pyi_private_set_type: Any

tests/typecheck/fields/test_base.yml

@@ -150,4 +150,18 @@
                     username = models.CharField(max_length=100)
 
                 class MyModel(AuthMixin, models.Model):
-                    pass
+                    pass
+-   case: can_narrow_field_type
+    main: |
+        from typing import cast, NewType
+        from django.db import models
+        Year = NewType("Year", int)
+        class Book(models.Model):
+            published = cast(models.Field[Year, Year], models.IntegerField())
+        book = Book()
+        reveal_type(book.published)  # N: Revealed type is 'main.Year*'
+        book.published = 2006  # E: Incompatible types in assignment (expression has type "int", variable has type "Year")
+        book.published = Year(2006)
+        reveal_type(book.published)  # N: Revealed type is 'main.Year*'
+        def accepts_int(arg: int) -> None: ...
+        accepts_int(book.published)